fix content rating options in admin form (bug 934735)

- Don't concatenate lazy strings in constants.
- Add waffling for ratingsbodies constants.
- Evaluate content ratings lazy strings when indexing Webapps.
- ratingsbodies constants tests.

![screen shot 2013-11-05 at 12 38 12 pm](https://f.cloud.github.com/assets/674727/1477377/349f3332-465a-11e3-9e41-1574178a23d0.png)

![screen shot 2013-11-05 at 12 38 09 pm](https://f.cloud.github.com/assets/674727/1477378/357536e4-465a-11e3-914d-0afc27508e2b.png)